Man began to add to the background radiation in the 1890s. In 1895, X
rays were discovered and since then have become increasingly useful in
medical diagnosis and therapy and in industry. In 1896, radioactivity
was discovered and radioactive substances were concentrated in
laboratories in order that they might be studied. In 1934, it was found
that radioactive forms of nonradioactive elements (_radioisotopes_)
could be formed and their use came to be widespread in universities,
hospitals, and industries.[4]
Then, in 1945, the nuclear bomb was developed. With the uranium or
plutonium fission that produces a nuclear explosion, there is an
accompaniment of intense gamma radiation. In addition, a variety of
radioisotopes are left behind in the form of the residue (_fission
fragments_) of the fissioning atoms. These fission fragments are
distributed widely in the atmosphere. Some rise high into the
stratosphere and descend (as _fallout_) over the succeeding months and
years.[5]
It is hard to try to estimate how much additional radiation is being
absorbed by human beings out of these man-made sources. Fallout is not
uniformly spread over the earth but is higher in those latitudes where
nuclear bombs have been most frequently tested. Then, too, people in
industries and research who are involved with the use of radioisotopes,
and people in medical centers who constantly deal with X rays, are
likely to get more exposure than others.
These adjuncts of modern science and medicine are more common and
widespread in technologically advanced countries than elsewhere, and
nuclear bombs have most often been exploded in just those latitudes
where the advanced countries are to be found.
Attempts have been made to work out estimates of this exposure. One
estimate, involving a number of technologically advanced countries
(including the United States) showed that an average of somewhere
between 0.02 and 0.18 rem per year was absorbed, as a result of
radiations (usually X rays) used in medical diagnosis and therapy.
Occupational exposure added, on the average, not more than 0.003 rem,
though the individuals constantly exposed in the course of their work
would naturally absorb considerably more than this overall average.
[Illustration: _Man-made radioactivity in the atmosphere produced this
nuclear-emulsion photograph. This radiation source is a fission product
produced in a nuclear explosion. The enlargement is 1200 diameters.
Compare this with the natural radioactivity depicted on page 28._]
On the whole, the highest absorption was found, as was to be expected,
in the United States.
If these findings are expanded to cover a 30-year period, assuming the
absorption will remain the same from year to year, it turns out that the
average absorption of man-made radiation in the nations studied varies
from 0.6 rem to 5.5 rems per 30 years per individual.
Considering the higher figure to be applicable to the United States, it
would seem that man-made radiation from all sources is now being
absorbed at nearly twice the rate that natural radiation is. To put it
another way, Americans are just about tripling their radiation dosage by
reason of the human activities that are now adding man-made radiation to
the natural supply. By far the major part of this additional dosage is
the result of the use of X rays in searching for decayed teeth, broken
bones, lung lesions, swallowed objects, and so on.